Filter duplicate check by date range for better performance

Optimize duplicate detection to only query existing transactions within the date range of the uploaded CSV file (plus 1-day buffer). This prevents loading the entire transaction history into memory when checking duplicates.

For example, uploading 2800 transactions from Jan-Mar 2024 will now only load existing transactions from that period rather than all historical transactions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

MoneyMap/Pages/Upload.cshtml.cs

@@ -255,19 +255,19 @@ namespace MoneyMap.Pages
             var previewItems = new List<TransactionPreview>();
             var addedInThisBatch = new HashSet<TransactionKey>();
 
-            // Load all existing transactions into memory for fast duplicate checking
+            // First pass: read CSV to get date range and all transactions
-            var existingTransactions = await _db.Transactions
+            var csvTransactions = new List<(TransactionCsvRow Row, Transaction Transaction, TransactionKey Key)>();
-                .Select(t => new TransactionKey(t.Date, t.Amount, t.Name, t.Memo, t.AccountId, t.CardId))
+            DateTime? minDate = null;
-                .ToHashSetAsync();
+            DateTime? maxDate = null;
 
-            using var reader = new StreamReader(csvStream);
+            using (var reader = new StreamReader(csvStream))
-            using var csv = new CsvReader(reader, new CsvConfiguration(CultureInfo.InvariantCulture)
+            using (var csv = new CsvReader(reader, new CsvConfiguration(CultureInfo.InvariantCulture)
             {
                 HasHeaderRecord = true,
                 HeaderValidated = null,
                 MissingFieldFound = null
-            });
+            }))
-
+            {
                 csv.Read();
                 csv.ReadHeader();
                 var hasCategory = csv.HeaderRecord?.Any(h => h.Equals("Category", StringComparison.OrdinalIgnoreCase)) ?? false;
@@ -284,6 +284,35 @@ namespace MoneyMap.Pages
                     var transaction = MapToTransaction(row, paymentResolution);
                     var key = new TransactionKey(transaction);
 
+                    csvTransactions.Add((row, transaction, key));
+
+                    // Track date range
+                    if (minDate == null || transaction.Date < minDate) minDate = transaction.Date;
+                    if (maxDate == null || transaction.Date > maxDate) maxDate = transaction.Date;
+                }
+            }
+
+            // Load existing transactions within the date range for fast duplicate checking
+            HashSet<TransactionKey> existingTransactions;
+            if (minDate.HasValue && maxDate.HasValue)
+            {
+                // Add a buffer of 1 day on each side to catch any edge cases
+                var startDate = minDate.Value.AddDays(-1);
+                var endDate = maxDate.Value.AddDays(1);
+
+                existingTransactions = await _db.Transactions
+                    .Where(t => t.Date >= startDate && t.Date <= endDate)
+                    .Select(t => new TransactionKey(t.Date, t.Amount, t.Name, t.Memo, t.AccountId, t.CardId))
+                    .ToHashSetAsync();
+            }
+            else
+            {
+                existingTransactions = new HashSet<TransactionKey>();
+            }
+
+            // Second pass: check for duplicates and build preview
+            foreach (var (row, transaction, key) in csvTransactions)
+            {
                 // Fast in-memory duplicate checking
                 bool isDuplicate = addedInThisBatch.Contains(key) || existingTransactions.Contains(key);